Exploration of the Dopamine Transporter: In Vitro and In Vivo Characterization of a High-Affinity and High-Specificity Iodinated Tropane Derivative (E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy- 3β-(4′-methylphenyl)nortropane (PE2I)

doi:10.1016/S0969-8051(97)00224-2

Nuclear Medicine and Biology

Volume 25, Issue 4, May 1998, Pages 331-337

https://doi.org/10.1016/S0969-8051(97)00224-2 Get rights and content

Abstract

For the diagnosis and follow-up of neurodegenerative diseases, many cocaine derivatives have been proposed as radioligands to explore the dopamine transporter. As none of them have all the criteria of specificity and kinetics for human use, we have developed a new derivative, (E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy-3β-(4′-methylphenyl)nortropane (PE2I), which displays promising properties. We report the characterization of PE2I in vitro on rat striatal membranes and in vivo in rats and in monkeys. PE2I had a high affinity (Kd = 0.09 ± 0.01 nM) and high specificity for the dopamine transporter. In rats we observed a high accumulation in the striatum; by contrast, a very low fixation was measured in the cortex. Moreover, a preinjection of a saturating dose of GBR 12909 prevented the striatal accumulation of PE2I by 74%. These results confirmed the specificity of PE2I for the dopamine transporter. In vivo in monkeys, SPECT studies showed a high accumulation in striatum. Moreover, an equilibrium state was obtained 1 h after injection. PE2I seemed to be the most promising ligand for the dopamine transporter exploration by SPECT using a single-day protocol.

Introduction

Several neurodegenerative diseases such as Parkinson’s disease are characterized by degeneration of the dopaminergic neurons resulting in a loss of dopaminergic transporters localized on the nerve endings in striatum. Nuclear medicine may play an important role in the diagnosis and follow-up of this disease; indeed, conventional anatomical exploration fails to detect any change in brain in this situation. In Parkinson’s disease, radiopharmaceuticals that bind to dopamine D2 receptors localized on the postsynaptic side may provide some information [24], but the most important change is observed on the presynaptic side. It has been demonstrated by both in vitro postmortem studies 17, 21and by PET [12]that there is a decrease in dopamine transporter sites during Parkinson’s disease. It can therefore be assumed that in vivo imaging of this transporter by SPECT exploration would be very useful for the diagnosis and evaluation of treatments. Cocaine is well-known for its ability to inhibit dopamine uptake by acting directly on the presynaptic dopamine transporter. Many iodinated cocaine derivatives have therefore been synthesized for SPECT, the most widely used being βCIT, and promising results have already been obtained with [¹²³I]βCIT in humans 1, 15, 25.

However, βCIT has two major problems. First, this ligand is not specific for the dopamine transporter; it also has a great affinity for the serotonin transporter [2]. Second, its kinetics are not very appropriate to human scintigraphy with 123-iodine. Indeed, the equilibrium state necessary to perform quantification is obtained only 24 h postinjection 7, 20. Many other derivatives have therefore been proposed to improve specificity and to obtain accurate kinetics, including FE-βCIT [18], FP-βCIT 5, 16, 23, IPT 14, 19, 22, and βCDIT [9]. Because none of these derivatives display optimum kinetics and specificity properties, we have developed a structure-activity relationship with two families of tropane where nitrogen and phenyl substitutions were performed in order to select the best ligand for SPECT application. From this study we have demonstrated that an iodopropenyl group link at the nitrogen bridgehead, associated with a 4′-methylphenyl link at C-3, provides the most promising tracer of these series [10]. We then selected (E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy-3β-(4′-methylphenyl)nortropane (PE2I) for further biological characterization. Thus, the aim of the present work was to characterize PE2I in vitro and in vivo. In vitro binding studies were evaluated on rat striatal membranes, and in vivo experiments were performed with biodistribution and autoradiographic studies in rats and with SPECT imaging in monkeys.

Section snippets

Radiolabelling Procedure

Both [¹²⁵I] and [¹²³I](E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy-3β-(4′-methylphenyl)nortropane (PE2I) were prepared by iododestannylation of the tributyltin precursor (Fig. 1). To a vial containing 50 μg of the stannyl precursor were added 50 μL EtOH, 50 μL HCl (0.1 N), 37 MBq [¹²⁵I]NaI (in 10 μL NaOH 0.1 N, specific activity: 75 TBq/mmol; Amersham, UK) or 74 MBq [¹²³I]NaI (in 100 μL NaOH 0.1 N, specific activity >185 TBq/mmol; Cis Bio, France) and 50 μL of 3% w/v hydrogen peroxide. The

Radiolabelling

The stannyl precursor of PE2I obtained by a method previously described by us [9]provided radioiodinated PE2I ([¹²⁵I]PE2I or [¹²³I]PE2I) with a greater than 50% yield. After purification by HPLC, radioiodinated compounds were obtained with high radiochemical purity (>95%). As these compounds were obtained without carrier, they displayed a high specific activity of 75 and >185 TBq/mmol for [¹²⁵I]PE2I and [¹²³I]PE2I, respectively. Chemical identity was checked after labelling by co-injection on

Discussion

The loss of neurons accompanying some neurodegenerative diseases such as Parkinson’s can be explored by dopamine transporter imaging. For this, Boja et al. proposed a cocaine congener, RTI-55 [3]. This promising ligand, also named βCIT, has been used in humans, and several teams employing it have observed a reduction in the uptake of this ligand in the caudate putamen of sufferers of Parkinson’s disease 1, 15. However, βCIT does not fulfil all the criteria of specificity and kinetics for wide

Acknowledgements

This work was supported by the Région Centre, Pôle GBM, Cis Bio International, and INSERM. We thank SAVIT for NMR and MS analyses; M. C. Furon and F. Wojciekowski for their technical assistance; and Doreen Raine for helpful editorial assistance.

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Original ArticlesExploration of the Dopamine Transporter: In Vitro and In Vivo Characterization of a High-Affinity and High-Specificity Iodinated Tropane Derivative (E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy- 3β-(4′-methylphenyl)nortropane (PE2I)

Abstract

Introduction

Section snippets

Radiolabelling Procedure

Radiolabelling

Discussion

Acknowledgements

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Anal. Biochem.

Eur. J. Pharmacol.

Imaging of dopamine transporters with iodine-123-β-CIT and SPECT in Parkinson’s disease

J. Nucl. Med.

High affinity binding of [125I]RTI-55 to dopamine and serotonin transporters in rat brain

Synapse

Practical benefit of [123I]FP-CIT SPET in the demonstration of the dopaminergic deficit in Parkinson’s disease

Eur. J. Nucl. Med.

SPECT imaging of dopamine and serotonin transporters with [123I]β-CITBinding kinetics in the human brain

J. Neural Transm.

Preparation and biological evaluation of iodine-125-IACFTA selective SPECT agent for imaging dopamine transporter sites

J. Nucl. Med.

A new iodinated tropane derivative (β-CDIT) for in vivo dopamine transporter explorationComparison with β-CIT

Synapse

Synthesis and ligand binding of nortropane derivativesN-substituted-2β-carbomethoxy-3β-(4′-iodophenyl)nortropane and N-(3-iodoprop-(2E)-enyl)-2β-carbomethoxy-3β-(3′,4′-disubstitutedphenyl)nortropane: New high affinity and selective compounds for the dopamine transporter

J. Med. Chem.

SPECT imaging of dopamine transporter sites in normal and MPTP-treated rhesus monkeys

J. Nucl. Med.

Original Articles
Exploration of the Dopamine Transporter: In Vitro and In Vivo Characterization of a High-Affinity and High-Specificity Iodinated Tropane Derivative (E)-N-(3-iodoprop-2-enyl)-2β-carbomethoxy- 3β-(4′-methylphenyl)nortropane (PE2I)

High affinity binding of [¹²⁵I]RTI-55 to dopamine and serotonin transporters in rat brain

Practical benefit of [¹²³I]FP-CIT SPET in the demonstration of the dopaminergic deficit in Parkinson’s disease

SPECT imaging of dopamine and serotonin transporters with [¹²³I]β-CITBinding kinetics in the human brain